1. Field of the Invention
The present invention relates to a slippage preventing apparatus for a vehicle and, more particularly, to a slippage preventing apparatus for a vehicle, which is provided with a traction control device and an automatic drive device, the traction control device being adapted to control an output of the engine in accordance with a degree of slippage of the driven wheels thereof.
2. Description of Related Art
Japanese Patent Publication (laid-open) No. 151,158/-1985 discloses a device for preventing the driven wheels from slipping by means of a so-called traction control which is designed to decrease an output of the engine during occurrence of slippage in accordance with a degree of slippage of the driven wheels which is determined on the basis of speeds of the driven wheels and the undriven wheels.
An automatic drive device is also known which is designed to control an output of the engine so as to maintain a given vehicle speed determined by command switches operated by the driven on the basis of signals from a vehicle speed determining section with the command switches.
It is accordingly to be noted that a slippage preventing apparatus for a vehicle may be designed from such a traction control device and an automatic drive device to thereby control the engine output, preventing slippage of the driven wheels. However, it is to be understood that the traction control device and the automatic drive device are adapted to control the engine output in different ways so that a combination of these devices may cause problems when they are operated at the same time. That is, when slippage occurs upon changes in road surface friction factors in a state of the automatic drive control in response to operation of the command switches, both the traction control device and the automatic drive device are brought into a state that they may be operated at the same time. In this case, the slippage should be controlled prior to maintaining the automatic drive, however, the traction control device is designed to reduce the engine output upon occurrence of slippage, thus slowing down a vehicle speed, while the automatic drive device is operated to maintain the driving state by increasing the engine output as the vehicle speed slows down. This difference may cause the traction control device to work inefficiently.